Because of the necessity for measuring the flow of fluid such as natural gas, in fluid transmission conduits such as pipelines, various types of flow meters have been devised for such fluid measurements. For example, turbine type flow meters and vortex shedding flow meters are only two examples of many different types of flow meters that have been developed. In order to insure the accuracy of such flow meters, various types of meter provers have been developed which provide for direct comparison of a volume of flowing fluid with a known volume of a calibrated meter prover. If the comparison yields a volumetric differential of zero or an acceptable variation therefrom, the flow meter is then said to be accurate within the limits of allowed tolerances. If the volumetric differential exceeds the limits allowed, then evidence is provided indicating that the flow meter may not be accurate. Flow meter inaccuracy may be temporary such as due to the passage of contaminants through the line or in the alternative, may be of a more permanent nature such as due to flow meter wear, obstruction, etc. In such cases, the flow meter must be removed from the line and repaired or replaced.
Numerous types of meter provers have been developed for verifying the accuracy of flow meters as exemplified by U.S. Pat. Nos. 3,605,810 of Moroney; 3,682,198 of Davis and 3,798,957 of Shannon, et al. Another U.S. patent of interest to this invention is U.S. Pat. No. 3,580,045 of Pfrehm, showing a bidirectional straight barrelled meter prover for cryogenic liquid and which is operated by a four-way spool valve. U.S. Pat. No. 3,254,523 of Fisher, et al. discloses a pulse generating apparatus responsive to piston passage in a bidirectional meter prover. U.S. Pat. No. 3,273,375 of Howe shows a straight type concontric chamber meter prover barrel and a rotary vane type four-way control valve. U.S. Pat. No. 4,419,880 discloses a meter prover having a driven piston which forces a known volume of fluid through a flow meter for volumetric comparison. U.S. Pat. Nos. 3,421,768 of Luse, et al.; 3,457,768 of Jasek and 3,120,118 of Boyle also show straight barrel type bidirectional provers with check valves providing unidirectional flow in certain connecting conduits between the control valve or valves and the calibrated barrel thereof.
To eliminate the necessity for sphere or piston handling devices in meter prover systems, various types of bidirectional meter provers have been developed and used for a considerable period of time. In bidirectional meter provers, a single fluid displacement element such as a piston or sphere is cycled back and forth within a calibrated meter prover barrel having a proving section therein defined by the spacing of a pair of detectors, typically detector switches. A conduit system is employed connecting the meter prover system with a flow line containing the flow meter to be proved. The conduit system incorporates a control valve or valves which control the direction of flow through the calibrated meter prover barrel. By manipulating the control valve to reverse the flow of fluid within the meter prover barrel, the displacer piston can be cycled back and forth within the calibration barrel. During movement of the displacer piston through the barrel, the detector switches are actuated by piston passage providing signals indicating measurement of a precise volume of fluid flow for comparison with the volume detected by the flow meter.
Although bidirectional meter provers have been found quite satisfactorily, nevertheless typical bidirectional provers have various shortcomings which limit their use in some cases. For example, it is sometimes desirable to provide meter provers which are of such small size as to be portable to thus enable a single meter prover system to be utilized for proving out remotely located flow meters. While some bidirectional meter provers have been developed that are sufficiently small as to be portable, the user must typically sacrifice from the standpoint of accuracy and functionality. Portable meter provers of bidirectional character have not heretofore been widely utilized primarily because of the requirement for long calibration barrels to insure desired meter prover accuracy. It is desirable therefore to provide a bidirectional meter prover mechanism which incorporates a meter prover barrel of minimal length and yet which provides the accuracy ordinarily found in bidirectional meter provers having calibration barrels of much longer length.
Most portable meter prover systems that have been developed require a significant amount of manual handling and control in order to insure reasonable accuracy of measurements, thereby causing labor costs to be significantly high as to render such systems noncompetitive and undesirable. It is desirable therefore to provide a compact bidirectional meter prover mechanism which may efficiency operated by an electronic controller with minimal manual handling and control to thus insure minimal labor costs for meter proving activity.
As mentioned above, it is desirable to provide a compact meter prover mechanism with a calibration barrel of minimal length. In most bidirectional meter prover systems, a significant length of meter prover barrel is required to compensate for inaccuracy during valve controlling movement between the flow direction controlling positions thereof. It is desirable therefore to provide a bidirectional meter prover mechanism wherein measuring flow through the calibrated barrel thereof occurs only when the control valve or valves are properly positioned.
If a single valve is employed in the meter prover system, it is typicalldy a four-way valve of either the rotary plug type or of the linear movable spool valve type. Multiple valves may also be employed which are operated simultaneously, or sequentially to reverse the direction of flow in the meter prover barrel. At any rate, the possibility of error in flow measurement during valve operation requires the meter prover barrel to be quite long in conventional bidirectional prover systems.
In conventional bidirectional meter provers, as the control valve begins its flow reversing movement, the displacer piston is immediately launched and begins its movement through the calibration barrel. A length of calibration barrel is provided for piston prerun to thus enable the valve to reach its fully reversed position before the piston member reaches a proving section within the barrel. The proving section is defined by spaced detector devices which detect passage of the piston to and from the proving section and provide appropriate signals to a controller system for processing. The proving section of the calibrated barrel has an accurately determined volume which is utilized electronically for comparison with the volume of flow detected during the same period by the meter being proved. Since the meter prover is bidirectional in character, a length of prover barrel must be provided at each end of the barrel for piston prerun during valve reversing operation. The typical length for a four inch meter prover with a twelve inch barrel is about forty-five feet. With a meter prover barrel of this length and the various lengths of conduit sections for establishing connection between the calibrated barrel and control valve, the entire meter prover system becomes of sufficient size that portability is not ordinarily possible. Therefore, such meter prover systems must be located at static positions and, of course, can accomplish proving only of flow meters located in close proximity thereto. For this reason, bidirectional flow meters are typically located at piping manifolds having multiple flow lines and are provided for selective connection to selected flow lines by valving systems of the manifold.
Various compact, short barrel flow meter provers have been developed in the recent past which can be truck mounted for example for desired portability. The general character of such meter provers is exemplified by an API Bulletin which shows a compact straight barreled meter prover system. In this case, the calibrated barrel is of such length as to provide for a certain length of piston prerun in order to compensate for inaccuracy of measurement during valve controlling movement. Also, bypass spools are provided at each end of the calibrated barrel and check valve controlled conduits control the flow of fluid to and from intermediate portions of the calibrated barrel. Meter provers of this character, though of somewhat compact nature, nevertheless require significantly long meter prover barrels as to render the portability thereof questionable.